1. Field of the Invention
This invention relates to the field of textile filter materials, and in particular to filter bags for removing corrosive liquid and solid contaminants from gaseous combustion exhaust.
2. Description of the Prior Art
Carbonized fabrics are known to be useful in a variety of applications requiring chemical and corrosion resistance. Suitable carbon fibers are made from pitch or other staple carbon sources, or can be prepared by thermal carbonization of synthetic fibers such as rayon, nylon, polyacrylonitrile (such as "Orlon"), or the like.
In preparing carbonized fibers from polyacrylonitrile, the fibers are slowly heated to approximately 250.degree.-300.degree. C. in air, at which controlled thermal oxidation and chemical transformation occurs. The fibers are held in this approximate temperature range (250.degree.-300.degree. C. or 480.degree.-570.degree. F.) for some minutes. In its oxidized and converted state, the fiber, now turned black, contains approximately 61% carbon. In this condition, the fiber is known as "oxidized polyacrylonitrile fiber" and is abbreviated "OPF".
Oxidized polyacrylonitrile is known in the art as a heat resistant fiber. Fabric of the material is particularly adapted to uses formerly reserved for asbestos. In particular, industrial and military protective and flame resistant clothing, flame resistant vehicle interior materials, electrical cable braiding and the like are examples of preferred uses for oxidized polyacrylonitrile. The material does not burn and is suitably flexible in a fabric for these uses.
Notwithstanding the foregoing uses as a flame retardant and heat resistant material, in order to provide chemically resistant fabrics for combustion exhaust filters and other corrosive environments, the prior art has gone one step further with carbon-containing materials such as oxidized polyacrylonitrile, in order to fully carbonize the fibers. Using further thermal pyrolysis at higher temperatures, OPF fibers are commonly "fully" carbonized into fibers containing at least 95% carbon. Although carbon fibers of this type are chemically resistant, the fibers are brittle and prone to deteriorate if flexed, abraded or otherwise stressed. These fibers are frequently used as filter elements and the like, both in activated and unactivated carbon states.
The present invention concerns the use of a precursor to the prior art's 95% carbon filaments, namely, the aforesaid oxidized polyacrylonitrile fiber, comprising approximately 61% carbon. The precursor material is preferably woven or knitted to form a filter bag through which combustion exhaust is passed, and upon which pollutants accumulate. The filter bag is strong and flexible, and may be periodically cleaned by automatic machinery as known in the art. The filter is resistant to chemical attack by sulfuric acid, hydrofluoric acid and other chemicals found in combustion exhaust application. The partial carbonization of the polyacrylonitrile fiber is found to provide adequate resistance to chemical deterioration due to abrasive and corrosive compounds and conditions found in combustion exhaust. To improve useful life, abrasion resistant coatings and lubricants can be applied to increase the mechanical strength of the fabric.